On a warm spring day in March, a young man turned up at a hospital in Peoria, scared to have found bright red blood in his urine and suffering from severe pain in his side.

Doctors at OSF St. Francis Medical Center were initially puzzled as to what could be causing such symptoms. Dr. Amar Kelkar noticed the man had what looked like black burn marks on his fingertips. The patient admitted the marks were from smoking synthetic marijuana.

As the day went on, more and more patients came in with uncontrolled bleeding and pain. It turned out they had one thing in common: They all had ingested “fake weed,” which refers to plants or oils with chemicals added that are meant to mimic the effects of cannabis.

Just days before, state officials had put out a warning that people were getting ill after ingesting synthetic cannabinoids in Chicago. Physicians in Peoria quickly realized they had a similar outbreak on their hands. Lab tests soon confirmed their suspicions: The drugs were laced with brodifacoum, a form of rat poison that prevents blood from clotting normally.

Statewide, from March through May, 164 such cases were reported, and four of the victims died after suffering strokes. The rest were saved by doctors who, after diagnosing the problem, administered plasma or vitamin K to stop the bleeding.

On Wednesday, some of those doctors published a report in the New England Journal of Medicine, retracing their treatment of the mysterious poisonings. With 80 similar cases recently reported in Wisconsin, including one death, and no single source identified, doctors worry that such outbreaks could continue.

Most commonly, patients had blood in their urine and bruises, but some also suffered bleeding from their noses or ears or in their stool, often accompanied by abdominal, joint and back pain; nausea; vomiting; and headaches.

Initially, doctors suspected liver disorders, because “poisoning is not on your radar,” said Kelkar, the paper’s lead author. But in those first days when patients came in with mysterious symptoms, doctors worked frantically to figure out the cause, ordering CAT scans, checking liver and kidney function, and submitting blood for lab tests.

“We were very much piecing this together in real time, looking to see if there was a pattern,” Kelkar said.

The doctors knew that warfarin, known by the brand name Coumadin, is commonly prescribed as a blood thinner for clotting disorders such as embolisms and strokes. But lab tests showed the imitation cannabis was laced with a type of “superwarfarin,” which has been commonly sold as rat poison since rats started developing a tolerance to warfarin.

Typically, synthetic marijuana is sold in small packages at convenient stores, under names such as K2, Spice and Scooby Snax. The product is hawked under numerous brand names as “herbal incense” or “potpourri,” and is also sold in gas stations, drug paraphernalia shops and on the streets.

Federal law prohibits certain synthetic cannabis compounds, but police say manufacturers try to tweak the chemical makeup to try to get around the prohibitions. Doctors suspect some dealers may have tried adding rat poison to increase the effects of the drug.

Even without rat poison, authorities warn that synthetic cannabinoids are unsafe and potentially life-threatening, as they can suppress breathing and brain function. In July, the U.S. Food and Drug Administration warned of the health risks from synthetic pot, which is associated with rapid heart rate, vomiting, violent behavior and suicidal thoughts.

The study was performed by members of the University of Illinois College of Medicine at Peoria and the Bleeding and Clotting Disorders Institute in Peoria, based on 34 patients at St. Francis Medical Center who were identified as having been poisoned.

Almost half of those patients reported daily use of synthetic marijuana, but none would provide a sample of the drug they used, and they didn’t want to say who supplied them with the drug, which made it harder for police to investigate.

Six patients included in the study were readmitted to the hospital, because they had left against medical advice and were unable to fill their prescriptions for vitamin K due to cost or shortage. About 1 out of 5 of the victims were regular drug users, many were poor and several were homeless.

Some of these patients engaged in “alarming” behaviors, the study reported, such as two who used the same batch of synthetic cannabinoids again after their initial treatment. One patient tried to donate plasma and was readmitted because of prolonged bleeding from the injection site. Because superwarfarin lasts so long in the body, it took patients two to three months to fully recover and required up to 15 vitamin K pills a day.

All four of the patients who died suffered intracranial bleeding, or strokes. One 37-year-old woman was brought unconscious to St. Francis by friends who said she’d been using synthetic cannabinoids and amphetamines. She was given intravenous vitamin K but suffered a stroke, and never recovered, according to the study.

Doctors speculated in their study that the poison could be used for bioterrorism, noting that the Department of Homeland Security classifies it as an “agent of opportunity” because of its commercial availability and toxicity.

In addition to alerting other doctors about how to recognize and treat such cases, co-author Dr. Jonathan Roberts, a hematologist at the Bleeding and Clotting Disorders Institute, said, “We hope that this would shed some light for local and national law enforcement, so that we can bring these people to justice and try to stop this from happening.”

That’s important because new illnesses have been reported elsewhere since Illinois’ cases emerged.

The problems were not confined to Illinois and Wisconsin. This year, the FDA reported, hundreds of people in about 10 states — many in the Midwest — were hospitalized for severe bleeding after ingesting the substance. The agency also warned against donating blood after using the drug.

Cloe Poisson/Hartford Courant

New Haven emergency responders tend to two people passed out on a bench across the street from City Hall on Aug. 16, 2018, in New Haven, Conn. A bad batch of synthetic marijuana was linked to dozens of overdoses in the area.

New Haven emergency responders tend to two people passed out on a bench across the street from City Hall on Aug. 16, 2018, in New Haven, Conn. A bad batch of synthetic marijuana was linked to dozens of overdoses in the area. (Cloe Poisson/Hartford Courant)

Last month near Yale University in Connecticut, at least 71 people were reported hospitalized from overdoses of synthetic marijuana. A published review last year found that synthetic marijuana was more dangerous than real cannabis because of frequent adverse effects.

The U.S. Centers for Disease Control and Prevention reported that in 2015, poison control centers nationwide received almost 8,000 calls for negative effects from synthetic pot.

Those who do get sick can also face numerous barriers to long-term treatment, the researchers reported, citing costs quoted to users of $24,000 to $34,000 a month.

While investigators have not identified any single supplier as the source of most of the poisonings, charges have been filed accusing some defendants of dealing the drug.

In April, three men were arrested and charged with selling the drug at the King Mini Mart in Chicago’s Lawndale neighborhood. A federal indictment charged the defendants with selling “FUB-AMB,” also known as AMB-FUBINACA, a synthetic cannabinoid that is a Schedule I controlled substance.

Also in April, a man allegedly found with 15 pounds of synthetic marijuana known as “Diablo” in his apartment in Pekin, Ill., was charged with drug-included homicide for the death of a man who smoked the drug in Peoria.

And a 71-year-old downstate woman and a California man were indicted this past spring on charges of conspiring to possess and distribute synthetic cannabis in Springfield.